1. Field of the Invention
The invention relates to an image forming apparatus for controlling image forming conditions regarding an image quality.
2. Related Background Art
FIG. 15 is a cross sectional view of a main section for explaining a construction of an image forming apparatus and shows the image forming apparatus for forming a color image by sequentially transferring developing agents of yellow, magenta, cyan, and black.
In the diagram, reference numeral 1 denotes a photosensitive drum as an image holding material. The drum 1 rotates clockwise toward the drawing. Reference numeral 2 denotes a charging unit to uniformly charge the surface of the photosensitive drum 1. The charging unit 2 is constructed by a charging line 2a and a grid 2b. Reference numeral 8 denotes a discharging lamp for exposing the whole surface of the drum 1, thereby allowing the surface potential of the drum 1 to approach a potential near 0 V at the front stage of the charging by the charging unit 2. Reference numeral 3 denotes an optical system comprising a laser light source, a collimating lens, a polygon mirror, and the like.
Reference numeral 4 denotes a developing unit to form a visible image from a latent image on the photosensitive drum 1 by a toner. Reference numeral 4M denotes a magenta developing unit; 4Y a yellow developing unit; 4C a cyan developing unit; and 4K a black developing unit. Reference numeral 5 denotes a cleaner to collect the toner remaining on the surface of the drum 1; 6 a transfer conveying apparatus (transfer drum) of the drum type for holding a recording material (transfer material) P by a drum-shaped transfer sheet and for transferring the toner image on the drum 1 onto the recording material P while conveying the recording material P; 7 a transfer charging unit; 10 a toner density sensor for detecting a mixture ratio of the toner and carrier for image formation which are stored in the developing unit 4; 11 a surface potential detector (potential sensor) for detecting the surface potential of the photosensitive drum 1; 17 a photoelectric sensor for reading an amount of reflected light of a test patch (test pattern) 22 which was irradiated onto the drum 1 by an LED illuminating apparatus 12; 16 a density converting circuit for converting a voltage generated from the photoelectric sensor 17 into density information; and 100 a control circuit for managing each of the above detection information and controlling each image forming condition.
The operation of each section will now be described hereinbelow.
First, when the surface of the photosensitive drum 1 is uniformly charged by the charging unit 2, a light image is exposed by the optical system 3. However, the light image is read by an original reading apparatus (scanner; not shown) and is exposed on the basis of the image processed signal. An electrostatic latent image is formed on the drum 1 by the exposure of the light image. The latent image is subsequently developed by the toner of the developing unit 4 and a visible image is derived. After that, the toner image is transferred to the recording material P by the transfer charging unit 7. The above processes are sequentially executed every four colors of yellow, magenta, cyan, and black, so that a full color image is obtained.
On the other hand, an output of the test patch 22 is indicated at a predetermined interval from the control circuit 100 and the test patch 22 is formed onto the drum 1 in a manner similar to the above processes. The test patch 22 is constructed by one or a plurality of patches having predetermined densities. A toner density of the test patch 22 detected by the photoelectric sensor 17 is arithmetically operated by the control circuit 100, thereby controlling image forming means, for example, a charging potential, a density conversion table (LUT), a toner density in the developing unit, a transfer current, and the like.
In the above kind of image forming apparatus, the above various kinds of controls for stabilization of the picture quality are executed at every interval during which a predetermined number of images are formed and each time a power source of the apparatus is turned on.
However, in case of executing a feedback control which changes developing conditions which exert an influence on the picture quality, in order to perform the control at a higher precision, it is necessary to further detect the image forming state in a micro state. According to the above construction, however, since the photoelectric sensor is used as a patch sensor, the reflected light amount of the whole test patch can be merely detected and there is a problem such that information which is effective to the feedback for setting the developing conditions cannot be obtained. Particularly, with respect to the defective reconstruction of a highlight portion which exerts a large influence on the image picture quality, information for the feedback control cannot be detected by the above test patch method.
On the other hand, at the timing for executing the above various kinds of controls for stabilization of the picture quality, a control for a change in state of the transfer material is not performed. Even when the toner density level of the toner image on the image holding material or the transfer drum is proper, when the paper in which a hygroscopic property actually changes is printed, the apparatus cannot rapidly made correspond to such a paper.
Further, in spite of the fact that the apparatus has characteristics such that the transfer efficiency is most easily changed in accordance with an environmental change of one day, there is also a problem such that when the conditions of the transfer efficiency are merely set on the basis of the number of transfer materials which are consumed, a clear image cannot stably be formed or the like.